1. Field of the Invention
The present invention relates to an X-ray CT (computed tomography) apparatus which includes a plurality of detecting elements two-dimensionally arranged for detecting X-rays, time-divides electric signals from the detecting elements by a switch, and reads-out the time-divided signals, a radiation detector and a method for reading out electric signals of a radiation detector.
2. Description of the Related Art
Conventionally, an X-ray CT apparatus generates a tomographic image by irradiating a patient as a object with X-rays, detecting the transmitted X-rays, and visualizing the inside structure of the object (see, for example, JP-A-2001-242253).
In an X-ray CT apparatus for medical use, the object is irradiated from an X-ray tube with X-rays in various directions, the transmitted X-rays are absorbed by a radiation detector at the facing position in the case of sandwiching the object, and an electric signal is finally generated. The electric signal reflects the strength of the transmitted X-ray. A tomographic image of the object is subjected to reconstruction based on data of the obtained signal, and is displayed on a display device.
For example, in a third-generation X-ray CT apparatus, an X-ray tube and a radiation detector are rotated on the plane vertical to the body axis of the object, the transmitted X-ray with which the object is irradiated from the X-ray tube is detected by the radiation detector, and a detected X-ray detecting signal is transmitted to a data acquisition system, thereby acquiring data. One tomographic image on the rotation plane is reconstructed by the rotation for one period (180° or 360°) and is then displayed.
Therefore, the radiation detector comprises a large number of detector blocks which are densely arranged along an arc on the rotation plane. The detector blocks are connected to the data acquisition system. The detector block is, e.g., a two-dimensional photodiode array detector block in many cases.
FIG. 17 is a diagram schematically showing one conventional two-dimensional photodiode array detector block. FIG. 18 is a side view showing a conventional two-dimensional photodiode array detector block 1 shown in FIG. 17. A scintillator is not shown in FIG. 17.
The two-dimensional photodiode array detector block 1 comprises a plurality of detecting elements 3 on a substrate 2 in the column direction (channel direction C), serving as the rotating direction of a radiator detector, and in the row direction (slice direction A), serving as the body axis, so as to acquire data for a plurality of tomographs in the data acquisition for one period.
The detecting element 3 comprises a scintillator 4 and a photodiode (PD) 5. Generally, the number of scintillators 4 is equal to the number of photodiodes 5, the X-rays incident on the scintillator 4 are converted into visible light, and the light is converted into an electric signal by the photodiode 5. Further, the electric signal covered by the photodiode 5 is captured from one end of the slice direction A serving as the body axis or both ends and is then guided to a data acquisition system (not shown) so as to arrange a large number of two-dimensional photodiode array detector blocks 1 on the rotation plane of the radiation detector.
Therefore, the photodiodes 5 are connected to a plurality of integrators by wire-bonding 6, and the electric signals from the photodiodes 5 are transmitted to integrators 7. Further, the photodiodes 5 are connected to a common switch 8 such as an MUX (Multiplexer) by wire-bonding 6, and the switch 8 is connected to a circuit substrate 9, such as an FPC (flexible printed circuit), on the substrate 2.
All detecting elements 3 are connected to the data acquisition systems with a one-to-one correspondence, thereby reducing active areas S1 of the detecting elements 3. However, wiring areas S2 increase in size and thus are not mounted on the substrate 2 by wiring. That is, the number of wire bonds 6 is limited. Then, the electric signals from the photodiodes 5 are stored into the integrators 7, are time-divided by the switch 8 in the slice direction A, and are sequentially outputted to the circuit substrate 9 such as the FPC. Further, the circuit substrate 9 guides the electric signals to the data acquisition system.
In addition, the increase of the number of the detecting elements 3 prevents a sufficient space for the detecting elements 3 from being ensured under the restriction on the wiring area S2. Then, another two-dimensional photodiode array detector block is proposed by improving the wiring pattern.
FIG. 19 is a schematic diagram showing another conventional two-dimensional photodiode array detector block which is formed by improving the wiring pattern. A scintillator is not shown in FIG. 19.
Referring to FIG. 19, a two-dimensional photodiode array detector block 1A comprises a plurality of detecting elements 3 which are two-dimensionally arranged in the form of a matrix on the substrate 2. A transistor switch 10 is arranged at the output of the photodiode 5 of the detecting element 3. The photodiodes 5 on the single column are connected to a common signal line 11 via the transistor switch 10. The transistor switch 10 of the photodiodes 5 on the single column is connected to a common control line 12.
In the two-dimensional photodiode array detecting element block 1A, X-rays are incident on the scintillator (not shown) of the detecting element 3 and then are converted into light. Further, the photodiode 5 converts the light into an electric signal, and the electric signal is stored into the photodiode 5 as a charge. The control line 12 sequentially transmits a switch control signal to the transistor switches 10 in the row direction and thus the transistor switches 10 become active. The electric signals are time-divided in parallel with each other from the photodiodes 5 on the same row, and are sequentially time-divided in the row direction (slice direction A) from the photodiodes 5 on the same column. That is, both the electric signals from the photodiodes 5 are outputted to the signal lines 11 via the transistor switches 10.
That is, in the two-dimensional photodiode array detector block 1A shown in FIG. 19, the transistor switches 10 are individually arranged at the photodiodes 5 and the signal lines 11 are commonly used, thereby reducing the number of signal lines 11.
FIG. 20 is a diagram showing a connecting method of the detecting elements 3 and a readout circuit in the conventional two-dimensional photodiode array detector block 1,1A. FIG. 21 is a schematic diagram showing a readout time of the electric signal from the conventional detecting elements 3 shown in FIG. 20.
The electric signals are outputted by the two-dimensional photodiode array detector block 1 or two-dimensional photodiode array detector block 1A in FIG. 17 or 19 having the detecting elements 3 corresponding to 16 rows, as shown in FIG. 20. One column is focused and then the electric signals from the detecting elements 3 are time-divided via a common integral amplifier 13. After that, the time-divided signals are A/D converted by an A/D converter 14 and are read-out by a readout circuit. Referring to FIG. 21, the readout time is used as the axis and then electric signals D from the 16 detecting elements 3 are sequentially read-out in row order by the readout circuit via the integral amplifier 13.
Then, the electric signals which are time-divided and read-out in parallel with each other in the row direction (slice direction A) from the detecting elements 3 of the column are transmitted to an image reconstructing unit via a circuit substrate and a data acquisition system. Further, the image reconstructing unit reconstructs a tomographic image of a object.
In the conventional two-dimensional photodiode array detector blocks 1 and 1A, the electric signals are time-divided depending on the number of rows of the detecting elements 3. Therefore, as the number of rows of the detecting element 3 is larger, it takes a longer time for reading-out the electric signals from the detecting elements 3 on all rows.
In particular, the integral amplifier 13 stores the electric signals from the detecting elements 3 as charges for a predetermined time for the purpose of integration. Therefore, the readout time of the electric signals from the detecting elements 3 is longer, mainly depending on the storing time of charges in the integral amplifier 13.
However, the time for reading-out the electric signals from the detecting elements 3 in the two-dimensional photodiode array detector block is generally limited to within a predetermined time. As the number of detecting elements increases for a predetermined readout-time of the electric signals, the readout time of the electric signals per row is shorter in accordance with the increased number of rows of the detecting element, that is, the electric signals need to be read-out faster.
If the radiation detector acquires the data 900 times per second, the data needs to be acquired once in a time of 1.111 ms. Since the number of rows of the detecting element is 16, the electric signals need to be read-out from the detecting elements corresponding to the 16 rows for a time of 1.111 ms, and the readout speed per row is thus 0.069 ms.
In accordance with the increase in readout speed of the electric signals, the readout circuit of the electric signal is not operated in the two-dimensional photodiode array detector block. Further, not only does the noise increase but also the image quality of the tomographic image generated by the above-obtained electric signal deteriorates. In other words, the number of rows of the detecting element arranged on the substrate of the two-dimensional photodiode array detector block is restricted.